U.S. patent number 6,998,192 [Application Number 10/652,118] was granted by the patent office on 2006-02-14 for negative electrode for a nonaqueous battery.
This patent grant is currently assigned to Quallion LLC. Invention is credited to Joanna Dodd, Phuong-Nghi Lam, Mikito Nagata, Hiroyuki Yumoto.
United States Patent |
6,998,192 |
Yumoto , et al. |
February 14, 2006 |
Negative electrode for a nonaqueous battery
Abstract
A negative electrode for use in secondary battery with
nonaqueous electrolyte having a high voltage and energy density and
a superior cycle property, characterized in that the active
material comprises composite carbon materials containing massive
ball-shaped graphite particles, carbon fibers, and graphite flakes.
The massive ball-shaped graphite particles provide porosity to the
composite, the carbon fibers improve packing density, conductivity,
and stiffness to prevent the body made thereof from swelling and
decomposing, and the graphite flakes reduce friction in the
mixture. An aqueous, non-fluorine-containing binder is used, along
with a titanium negative substrate.
Inventors: |
Yumoto; Hiroyuki (Stevenson
Ranch, CA), Dodd; Joanna (Burbank, CA), Nagata;
Mikito (Valencia, CA), Lam; Phuong-Nghi (Burbank,
CA) |
Assignee: |
Quallion LLC (Sylmar,
CA)
|
Family
ID: |
35767872 |
Appl.
No.: |
10/652,118 |
Filed: |
August 28, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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60406846 |
Aug 29, 2002 |
|
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|
Current U.S.
Class: |
429/231.4;
429/231.8; 429/231.1; 429/212; 429/232; 429/245; 252/182.1 |
Current CPC
Class: |
H01M
4/364 (20130101); H01M 4/622 (20130101); H01M
4/133 (20130101); H01M 10/0525 (20130101); H01M
4/1393 (20130101); H01M 4/485 (20130101); H01M
2004/027 (20130101); Y02E 60/10 (20130101); H01M
4/525 (20130101); H01M 4/0404 (20130101); H01M
4/505 (20130101); H01M 4/661 (20130101) |
Current International
Class: |
H01M
4/48 (20060101) |
Field of
Search: |
;429/231.4,231.8,232,212,231.1,245 ;252/182.1 |
References Cited
[Referenced By]
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Other References
Hitachi Chemical Website-Anode Material for Lithium Ion Batteries
section. cited by examiner .
Mesocarbon Microbeads Removes.RTM. M Series , MCMB,
http://www.ashland-suedchemie.de/en/english/c.sub.--produkte/cprodukteMes-
ocarbon Microbeads Removes.RTM. M Series, MCMB,
http://www.ashland-suedchemie.de/en/english/c.sub.--produkte/cprodukte5.h-
tm. cited by other.
|
Primary Examiner: Weiner; Laura
Attorney, Agent or Firm: Gavrilovich, Dodd & Lindsey
LLP
Parent Case Text
REFERENCE TO PRIOR FILED APPLICATIONS
This application is related to U.S. patent application Ser. No.
10/264,870 filed on Oct. 3, 2002 and entitled "Negative Electrode
for a Nonaqueous Battery," which claims priority to U.S.
Provisional Patent Application Ser. No. 60/406,846, filed on Aug.
29, 2002 and entitled "Negative Electrode for a Nonaqueous
Battery," each of the which is assigned to the assignee of the
current application and is hereby incorporated by reference herein
in its entirety.
Claims
What is claimed is:
1. A negative electrode, comprising: a substrate; and a coating on
the substrate, the coating including a binder and a a carbonaceous
material that includes ball-shaped graphite particles, carbon
fibers, graphite flakes, wherein the ball shaped graphite particles
include smaller graphite particles arranged such that the ball
shaped graphite particles are isotropic.
2. The negative electrode of claim 1, wherein the carbonaceous
material includes a mixture of 10 90% ball-shaped graphite
particles, 7.5 80% carbon fibers, and 2.5 30% graphite flakes by
weight.
3. The negative electrode of claim 1, wherein the carbonaceous
material includes a mixture of 10 80% ball-shaped graphite
particles, 15 80% carbon fibers, and 2.5 30% graphite flakes by
weight.
4. The negative electrode of claim 1, wherein the carbonaceous
material includes a mixture of approximately 80% ball-shaped
graphite particles, 15% carbon fibers, and 5% graphite flakes by
weight.
5. The negative electrode of claim 1, wherein the ball-shaped
graphite particles have an average particle size of 10 35 .mu.m,
the carbon fibers have an average particle size of 10 35 .mu.m, and
the graphite flakes have an average particle size of 10 35
.mu.m.
6. The negative electrode of claim 1, wherein the binder is
water-based.
7. The negative electrode of claim 1, wherein the binder does not
contain fluorine.
8. The negative electrode of claim 1, wherein the binder includes
carboxymethyl cellulose.
9. The negative electrode of claim 8, wherein the binder includes
styrene butadiene rubber.
10. The negative electrode of claim 9, wherein the styrene
butadiene includes 0 5% of the total weight of binder plus
carbonaceous material.
11. The negative electrode of claim 9, wherein the substrate
includes titanium.
12. The negative electrode of claim 8, wherein the carboxymethyl
cellulose includes 0 10% of the total weight of binder plus
carbonaceous material.
13. The negative electrode of claim 1, wherein the substrate
includes titanium.
14. The electrode of claim 1, wherein the smaller graphite
particles are unorganized in the ball shaped graphite
particles.
15. A battery, comprising: a case; a negative electrode housed in
the case, the negative electrode having a negative coating on a
negative substrate, the negative coating having a first binder and
a carbonaceous material that includes ball-shaped graphite
particles, carbon fibers, and graphite flakes, wherein the ball
shaped graphite particles include smaller graphite particles
arranged such that the ball shaped graphite particles are
isotropic.
16. The battery of claim 15, wherein the carbonaceous material
includes 10 90% ball-shaped graphite particles, 7.5 80% carbon
fibers, and 2.5 30% graphite flakes by weight.
17. The battery of claim 15, wherein the carbonaceous material
includes 10 80% ball-shaped graphite particles, 15 80% carbon
fibers, and 2.5 30% graphite flakes by weight.
18. The battery of claim 15, wherein the carbonaceous material
includes approximately 80% ball-shaped graphite particles, 15%
carbon fibers, and 5% graphite flakes by weight.
19. The battery as in claim 15, wherein the case is hermetically
sealed.
20. The battery as in claim 15, wherein the first binder is
water-based.
21. The battery as in claim 15, wherein the first binder contains
no fluorine.
22. The battery as in claim 15, wherein the first binder includes
carboxymethyl cellulose.
23. The battery as in claim 22, wherein the first binder further
includes styrene butadiene rubber.
24. The battery as in claim 23, wherein the negative substrate
includes titanium.
25. The battery as in claim 14, wherein the negative coating has a
porosity of 20 45%.
26. The battery as in claim 15, further comprising: a positive
electrode housed in the case, the positive electrode having a
positive coating on a positive substrate, wherein the positive
coating has a porosity of 20 40%.
27. The battery as in claim 15, wherein the negative electrode
forms C.sub.6Li.sub.n, and at a maximum state of charge,
0.5.ltoreq.n.ltoreq.0.9.
28. The battery as in claim 15, further comprising: a positive
electrode housed in the case, wherein the positive electrode is
constructed so as to form Li.sub.1-pMO.sub.2 during operation of
the battery, wherein M includes one or more transition metals, and
at a maximum state of charge, 0.6.ltoreq.p.ltoreq.0.8.
29. The battery as in claim 15, wherein the negative substrate
includes titanium.
30. The battery as in claim 29, further comprising: an electrolyte
in the case and activating the negative electrode and a positive
electrode, wherein the electrolyte includes a lithium salt in a
cyclic and linear solvent.
31. The battery of claim 15, wherein the smaller graphite particles
are unorganized in the ball shaped graphite particles.
32. A method for making a negative electrode includes the steps of:
providing a substrate; combining components that include
ball-shaped graphite particles, carbon fibers, graphite flakes, and
a binder in a solvent, wherein the ball shaped graphite particles
include smaller graphite particles arranged such that the ball
shaped graphite particles are isotropic; mixing the components to
form a slurry; coating at least a portion of the substrate with the
slurry; and evaporating the solvent.
33. The method of claim 32, wherein the substrate includes
titanium.
34. The method of claim 32, wherein the solvent is water.
35. The method of claim 32, wherein the binder contains no
fluorine.
36. The method of claim 32, wherein the binder includes
carboxymethyl cellulose.
37. The method of claim 36, wherein the binder further includes
styrene butadiene.
38. The method of claim 37, wherein the substrate includes
titanium.
Description
GOVERNMENT LICENSE RIGHTS
None
FIELD
This invention relates to a negative electrode for a nonaqueous
battery and more particularly to a negative electrode having
carbonaceous active material.
BACKGROUND
Aerospace devices and implantable medical devices such as
pacemakers, defibrillators, speech processors, left ventricular
assist devices (LVAD), and neurostimulators have many stringent
requirements. They must be small and lightweight, and must
therefore have high energy density batteries to provide adequate
capacity with long cycle life and long calendar life. For aerospace
applications, good low temperature performance is also needed, and
for implantable medical devices, good body temperature performance
is also needed.
SUMMARY
A negative electrode is disclosed, comprising: a substrate; and a
coating on the substrate comprising: a carbonaceous material
comprising a mixture of massive ball-shaped graphite particles,
carbon fibers, and graphite flakes; and a binder. The substrate may
comprise titanium. The carbonaceous material may comprise a mixture
of 10 90% massive ball-shaped graphite particles, 7.5 80% carbon
fibers, and 2.5 30% graphite flakes by weight, and preferably
comprises a mixture of approximately 80% massive ball-shaped
graphite particles, 15% carbon fibers, and 5% graphite flakes by
weight. The massive ball-shaped graphite particles, carbon fibers,
and graphite flakes may have an average particle size of 10 35
.mu.m. The binder may be water-based. A binder may be used that
contains no fluorine. The binder may comprise carboxymethyl
cellulose and may additionally comprise styrene butadiene rubber,
which may comprise 0 5% of the total weight of binder plus
carbonaceous material. The carboxymethyl cellulose may comprise 0
10% of the total weight of binder plus carbonaceous material.
A battery is disclosed, comprising: a case; an electrode assembly
housed in the case and comprising: a negative electrode comprising:
a negative substrate; and a negative coating on the negative
substrate comprising: a carbonaceous material comprising a mixture
of massive ball-shaped graphite particles, carbon fibers, and
graphite flakes; and a first binder; a positive electrode
comprising: a positive substrate; and a positive coating on the
positive substrate comprising: a lithium metal oxide; and a second
binder; a separator between the negative and positive electrodes;
an electrolyte. The case may be hermetically sealed. The negative
substrate may comprise titanium. The first binder may be
water-based. A first binder may be used that contains no fluorine.
The first binder may comprise carboxymethyl cellulose and may
further comprise styrene butadiene rubber, and the negative
substrate may comprise titanium. The negative coating may have a
porosity of 20 45%. The positive coating may have a porosity of 20
40%. The negative electrode may form C.sub.6Li.sub.n, and at a
maximum state of charge, n may be from 0.5 to 0.9. The positive
electrode may form Li.sub.1-pMO.sub.2, wherein M comprises one or
more transition metals, and at a maximum state of charge, p may be
from 0.6 to 0.8. The electrolyte may comprise a lithium salt in a
cyclic and linear solvent.
A method for making a negative electrode is disclosed, comprising
the steps of: providing a substrate; combining massive ball-shaped
graphite particles, carbon fibers, graphite flakes, and a binder in
a solvent; mixing to form a slurry; coating at least a portion of
the substrate with the slurry; and evaporating the solvent. The
substrate may comprise titanium. The solvent may be water. A binder
may be used that contains no fluorine. The binder may comprise
carboxymethyl cellulose and may further comprise styrene
butadiene.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic, longitudinal sectional view of a battery of
the present invention.
FIG. 2 is a scanning electron microscope photograph of a
carbonaceous composite material of the present invention comprising
spheres, fibers, and flakes.
FIG. 3 is a graph showing the cycle life of the present invention
at 37.degree. C.
FIG. 4 is a graph showing the cycle life of the present invention
at low temperature.
FIG. 5 is a graph showing the calendar life of the present
invention at body temperature.
DETAILED DESCRIPTION
The following text describes the preferred mode presently
contemplated for carrying out the invention and is not intended to
describe all possible modifications and variations consistent with
the spirit and purpose of the invention. The scope of the invention
should be determined with reference to the claims.
This invention provides a negative electrode that improves low
temperature cycle life of a secondary battery having a nonaqueous
electrolyte. The present invention also provides a secondary
battery with nonaqueous electrolyte, having high voltage, and
excellent cycle properties and calendar life. The invention also
provides a negative electrode substrate that minimizes
capacity-reducing side reactions between the substrate and
electrolyte. The invention also provides a negative electrode
having high conductivity to improve cycle life. The invention also
provides a negative electrode having a high packing density, which
improves conductivity, thereby improving cycle life, and increases
energy density, thereby reducing battery volume and weight. The
invention also provides a negative electrode having a binder
comprising carboxy methylcellulose (CMC) and styrene butadiene
rubber (SBR) to increase safety.
A negative electrode is provided having features selected to
improve the low temperature cycle life of a secondary battery
having a nonaqueous electrolyte. The negative electrode of the
present invention preferably comprises a coating including a
carbonaceous material comprising a mixture of massive ball-shaped
graphite particles, carbon fibers, and graphite flakes. Preferably,
the massive ball-shaped graphite particles, carbon fibers, and
graphite flakes have an average particle size of 10 30 .mu.m, and
occur in a ratio of approximately 80% massive ball-shaped graphite
particles: 15% carbon fiber: 5% graphite flakes. These particles
may be a combination of natural and artificial graphite. This
composite graphite material provides improved low temperature cycle
life, high conductivity, and high stiffness, which reduces
swelling.
When an unstable fluorine-containing binder such as PVdF is present
in a negative electrode active layer, Li+is consumed in the
reaction F.sup.-+Li.sup.+.fwdarw.LiF. To avoid this, the negative
electrode coating of the present invention preferably contains a
binder that does not contain fluorine, and more preferably
comprises carboxymethyl cellulose (CMC), which may include Na-CMC,
NH.sub.4-CMC, or a mixture thereof. The binder may additionally
comprise styrene butadiene rubber (SBR), which imparts elasticity
to the coating. The CMC preferably comprises 0 10% of the total
weight of binder plus carbonaceous material, and the SBR preferably
comprises 0 5% of the total weight of binder plus carbonaceous
material. Alternatively, another non-fluorine-containing binder or
a fluorine-containing binder that is more stable than PVdF in the
battery's operating range may be used to prevent cell degradation,
thus improving calendar life and safety.
In one preferred embodiment of the invention, the negative
electrode comprises a titanium substrate that is coated with a
slurry of carbonaceous material comprising a mixture of massive
ball-shaped graphite particles, carbon fibers, and graphite flakes
and a non-fluorine-containing, water-based binder comprising CMC
and SBR.
In a preferred method for making a negative electrode, massive
ball-shaped graphite particles, carbon fibers, and graphite flakes,
a binder, and water are mixed together to form a slurry. A titanium
substrate is coated with the slurry; then the water is removed by
evaporation. Preferably, the binder used is CMC or CMC+SBR.
FIG. 1 is a schematic view of a battery 10 of the present
invention. An electrode assembly 12 is housed in a case 14, which
is preferably hermetically sealed. Hermetic sealing is advantageous
because once impurities in the battery are consumed, no more
impurities can enter the inside of the battery. Thus, any lithium
consumption and active material degradation reactions due to
impurities are ended, whereby battery capacity is stabilized and
does not degrade further due to any impurities. The electrode
assembly 12 comprises positive electrode 16 and negative electrode
20, separated by a separator 18, and an electrolyte 19. Positive
electrode 16 comprises a positive substrate 60 having a coating 62
made of a lithium metal oxide 64 and a binder 66. The positive
substrate 60 is preferably aluminum. The lithium metal oxide is
preferably a lithium transition metal oxide, and more preferably
LiCo.sub.xNi.sub.yM.sub.zO.sub.2, where M=Mn, Al, Li, Sn, In, Ga,
or Ti, and 0.15.ltoreq.x.ltoreq.0.5, 0.5.ltoreq.y.ltoreq.0.8, and
0.ltoreq.z.ltoreq.0.15, and most preferably
LiCo.sub.xNi.sub.yAl.sub.zO.sub.2 where 0.15.ltoreq.x.ltoreq.0.5,
0.5.ltoreq.y.ltoreq.0.8, and 0.ltoreq.z.ltoreq.0.05. The binder is
preferably PVdF. Coating 62 preferably also contains a material for
enhancing conductivity, such as acetylene black or graphite.
The separator 18 is preferably a polyolefin, such as
polyethylene.
The electrolyte 19 is not particularly limited and may be an
organic liquid, polymer, or inorganic. An electrolyte is chosen
that allows reversible lithium intercalation. The electrolyte is
preferably a lithium salt in a cyclic and linear solvent. The
electrolyte is more preferably 1-M to 1.2-M LiPF.sub.6 in 25 30%
ethylene carbonate (EC) and 70 75% diethyl carbonate (DEC).
The negative electrode 20 of the present invention comprises
negative substrate 22 coated with a slurry 30 comprising a
carbonaceous mixture 40 and a binder 50. Binder 50 preferably is
fluorine free and more preferably comprises CMC 52. Binder 50 may
additionally contain SBR 54.
The negative electrode preferably contains active material in an
amount such that its maximum n in C.sub.6Li.sub.n is n=0.5 to 0.9,
and most preferably n=0.8. Stated another way, when the battery is
fully charged (preferably, to 4.1 to 4.2 V), the lithium has
intercalated at the negative electrode to form C.sub.6Li.sub.0.8.
When n is too high, electrolyte reduction occurs, and the
interlayer spacing is increased and swelling and contraction of the
material becomes more pronounced. These mechanisms tend to reduce
battery calendar life. On the other hand, when maximum n is too
low, the battery voltage and capacity are low. n=0.5 to 0.9, and
most preferably n=0.8 has been found to be a good compromise
between calendar life and battery voltage and capacity. The
positive electrode preferably forms Li.sub.1-pMO.sub.2, wherein M
comprises one or more transition metals, and at a maximum state of
charge, p=0.6 to 0.8, and most preferably p=0.7. When the battery
is fully charged, the lithium has deintercalated at the positive
electrode to form, most preferably,
Li.sub.0.3Co.sub.xNi.sub.yAl.sub.zO.sub.2, where
0.15.ltoreq.x.ltoreq.0.5, 0.5.ltoreq.y.ltoreq.0.8, and
0.ltoreq.z.ltoreq.0.05. When p is too high, electrolyte oxidation
occurs.
FIG. 2 shows a scanning electron microscope photograph
(.times.1000) of the preferred carbonaceous mixture 40 of the
present invention. It preferably contains massive ball-shaped
graphite particles 42, carbon fibers 44, and graphite flakes 46.
Preferably, the massive ball-shaped graphite particles, carbon
fibers, and graphite flakes have an average particle size of <40
.mu.m, and occur in a ratio of approximately 80% massive
ball-shaped graphite particles: 15% carbon fibers: 5% graphite
flakes.
Massive ball-shaped graphite particles 42 are made up of smaller
graphite particles that are synthesized together into an
unorganized isotropic ball-shaped structure. Since the smaller
graphite particles are unorganized, the massive graphite particles
are very porous, and there are more paths through which lithium can
diffuse. The ball-shaped particles are made up of high
crystallinity graphite, resulting in a high capacity. The capacity
of the massive ball-shaped graphite particles is 360 mAh/g, with 35
mAh/g being irreversible. Massive ball-shaped graphite particles 42
are preferably very porous with a high surface area of preferably
about 3.8 m.sup.2/g, as measured by BET, which allows easier
lithium diffusion throughout the particles, resulting in higher
rate capability and better performance at low temperatures. Massive
ball-shaped graphite particles 42 preferably have a real density of
preferably 2.0 2.3 g/cc, and more preferably about 2.24 g/cc, and
an average particle size of preferably 10 35 .mu.m and more
preferably about 20.6 .mu.m. Massive ball-shaped graphite particles
42 are available from Hitachi Chemical under the trade name MAG D.
These particles help provide porosity to the carbonaceous mixture,
which is important for allowing the electrolyte 19 to contact the
surface of the carbon and to react with it.
Carbon fibers 44 preferably have a specific surface area of <5
m.sup.2/g, an average particle size of preferably <40 .mu.m and
more preferably 10 35 .mu.m, a d002 (layer distance) of <3.36
.ANG., and an Lc of >100 nm. Carbon fibers that are too long may
cause microshorts by penetrating the separator that separates the
positive and negative electrodes. The addition of the carbon fibers
to the carbonaceous composition improves packing density and
conductivity. Carbon fibers also may intensify the stiffness of the
anode and thus prevent the anode body from swelling and
decomposing. The carbon fiber used in the invention may be a vapor
grown carbon fiber. The carbon fiber may be prepared by subjecting
hydrocarbons such as benzene, methane, propane, and so on to vapor
phase heat-decomposition under the presence of catalyst base plate
made of Fe, Ni, Co, and so on in order to make carbon fibers
deposit and grow on the base plate. Other examples are pitch carbon
fibers, made from petroleum or coal pitch as a raw material through
a spinning and carbonating treatment, and carbon fibers made from
polyacrylonitrile (PAN), which may be used in the invention.
Natural or artificial graphite flakes 46 are soft and tend to
reduce friction in the mixture because the planes of carbon can
slip with respect to one another, allowing the graphite flakes 46
to fit within the spaces in the mixture. We prefer an average
particle size of preferably <40 .mu.m and more preferably 10 35
.mu.m.
A mixture of carbon fiber and graphite flakes is available in the
most preferred ratio of 75% carbon fiber to 25% graphite flakes
under the trade name MELBLON MILLED FIBER FM70 available from
Petoca Materials Ltd.
In the present invention, the mixture ratio between the massive
ball-shaped graphite particles, the carbon fibers, and the graphite
flakes is an important factor. The massive ball-shaped graphite
particle content of the composite carbon material is preferably
from 10 wt. % to 90 wt. %; the carbon fiber content of the
composite carbon material is preferably from 7.5 wt. % to 80 wt. %;
and the graphite flake content of the composite carbon material is
preferably from 2.5 wt. % to 30 wt. %. If the massive ball-shaped
graphite particle content is above 90 wt. %, the surface contact
between the particles, and thus conductivity and capacity, is too
low. If the fiber content is above 80 wt. %, the packing density,
and thus the capacity, is decreased. If the flake content is above
30 wt. %, the surface area is too high, and therefore the amount of
lithium consumed in forming the SEI layer is too great, thereby
reducing capacity. Also, a flake content above 30 wt. % may lack
the structural support to keep the pores open to keep the porosity
within an optimal range to allow the electrolyte to react freely
with the surface of the carbonaceous material. The negative coating
preferably has a porosity of 20 45%, and the positive coating
preferably has a porosity of 20 40%.
The binder 50 of the negative active material coating preferably
contains no fluorine, and more preferably comprises CMC. Even more
preferably, styrene butadiene rubber (SBR) is added, which imparts
elasticity to the mixture. In contrast, prior art electrodes
contained a PVdF binder, which was unstable and tended to break
down, especially at higher temperatures, consuming Li+ in the
reaction F.sup.-+Li.sup.+.fwdarw.LiF. To avoid this, the negative
electrode coating of the present invention preferably uses a
CMC+SBR binder containing no fluorine. Alternatively, another
non-fluorine-containing binder or a fluorine-containing binder that
is more stable than PVdF in the battery's operating range may be
used. Another advantage of using a binder containing SBR as
compared with PVdF is that SBR binds to more area of the graphite
mixture than does PVdF; therefore, the exposed surface area of
graphite is minimized, minimizing electrolyte decomposition at the
graphite surface. A dispersion in water of the carbonaceous mixture
(described above), CMC, and SBR can be made to form a slurry that
can be conveniently coated onto to a metal foil substrate.
In a preferred embodiment, the substrate is about 12 .mu.m thick
and is preferably titanium or an appropriate alloy thereof, but may
alternatively comprise other substrates such as copper or stainless
steel. A tape test was used to determine adhesion of a coating
comprising the graphite active material of the present invention
and SBR and CMC binders (96:2.5:1.5) to commercially pure titanium
(CP Ti) and to copper substrates. In that test, test samples were
made by coating each substrate with the graphite coating, drying
it, then cutting through the coating with a knife to form a grid 10
mm.times.10 mm grid pattern, with lines in the grid 1 mm apart.
Scotch brand tape from 3M was then applied to the coating, and then
peeled away. The coating was found to adhere better to the titanium
substrate than to the copper.
The preferred ratios of carbon materials in the coating are:
TABLE-US-00001 more most preferably preferably preferably massive
ball-shaped graphite particles 10 90% 10 80% about 80% carbon
fibers 7.5 80% 15 80% about 15% graphite flakes 2.5 30% 2.5 30%
about 5% total 100%
To this carbonaceous combination, binder materials are added in the
following preferred mass per cents:
TABLE-US-00002 more most preferably preferably preferably CMC 0 to
30% 0 to 10% about 1.5% SBR 0 to 30% 0 to 5% about 2.5%
To make a negative electrode, a mixture of shapes of carbon
particles, a binder, and water are mixed together to form a slurry,
which is applied to both sides of metal foil, then dried. In a
preferred method, massive ball-shaped graphite particles, carbon
fibers, and graphite flakes in the amounts described above are
first combined. Then the binder, preferably CMC, 2% in water, is
added and mixed. Following that, SBR, 40% in water, is preferably
added with additional water, then mixed to form a slurry having the
mass per cents of CMC and SBR as indicated above. A 12-.mu.m
titanium foil substrate is coated with the slurry, then dried by
evaporating the water off using heat, then rolled.
FIG. 3 is a graph showing the battery cycle life of the present
invention at 37.degree. C. compared with a battery using a
different negative electrode. In the battery of the present
invention, the electrolyte was 1.2-M LiPF6 in 25% EC/75% DEC, the
CMC/SBR/carbon mass ratio was 1.5:2.5:96, the ratio of the three
types of carbon was 80% massive ball-shaped graphite particles: 15%
carbon fibers: 5% graphite flakes, and the substrate was copper.
The porosity of the negative coating was 32%. The battery used for
comparison ("Comparison Battery A") had the same positive active
materials, but had a negative electrode composed of a mixed
graphite not having massive ball-shaped particles or carbon fibers,
and having a PVdF binder. Three Test Batteries and one Comparison
Battery A were cycled at a rate of 0.5 C at 37.degree. C. and the
capacity was recorded. The three Test Batteries were cycled between
2.7 V and 4.2 V, while the Comparison Battery A was cycled between
2.5 V and 4.1 V. The average capacity retention for the three
batteries of each type is plotted against cycle number. The
capacity retention after 500 cycles was significantly better for
the average Test Battery (.about.90%) than for the average
Comparison Battery A (.about.73%).
FIG. 4 is a graph showing the battery cycle life of the present
invention at a temperature of 10.degree. C. compared with a battery
using a different mixture of carbon particles as described in U.S.
patent application Ser. No. 10/264,870 filed Oct. 3, 2002, assigned
to the assignee of the present invention and incorporated herein by
reference. The test battery was as described above. The battery
used for comparison ("Comparison Battery B") was similar to the
Test Battery, except that the carbon mixture was 70% hard spheres:
22.5% carbon fibers: 7.5% graphite flakes. Three batteries of each
type were cycled at a rate of 0.5 C at 10.degree. C., and the
capacity was recorded. The average capacity retention for the three
batteries of each type is plotted against cycle number for every
200 cycles. The capacity retention after 800 cycles was
significantly better for the average Test Battery (.about.84%) than
for the average Comparison Battery B (.about.52%).
FIG. 5 is a graph showing the calendar life of the present
invention at body temperature. The Test Battery was as described
above, and the battery used for comparison was Comparison Battery A
described above. The Test Batteries were charged to 4.2 V, then
stored at 37.degree. C.; the Comparison Batteries were charged to
4.1 V, and then stored at 37.degree. C. The negative potential was
approximately 0.1 V. The batteries were tested monthly for capacity
retention. After 6 months, the capacity retention of the average
Test Battery was significantly better (.about.92%) than that of the
average Comparison Battery A (.about.81%), even though they were
subjected to harsher storage conditions of 4.2 V compared to 4.1 V
of the Comparison Batteries.
While the invention herein disclosed has been described by means of
specific embodiments and applications thereof, numerous
modifications and variations could be made thereto by those skilled
in the art without departing from the scope of the invention set
forth in the claims. Furthermore, various aspects of the invention
may be used in other applications than those for which they were
specifically described herein.
* * * * *
References